This application is based upon and claims benefit of priority of Japanese Patent Application No. 2000-392792 filed on Dec. 25, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a pressure sensor having a semiconductor sensor chip for converting a strain of a diaphragm sensing a pressure applied thereto into an electrical signal.
2. Description of Related Art
An example of conventional pressure sensors of this kind is shown in FIG. 11. FIG. 11 shows a cross-sectional view of a pressure sensor used for detecting a high fuel pressure in a fuel injection system (e.g., in a common rail injection system) of an automobile. A metallic stem 10 having a diaphragm 11 for sensing a pressure applied thereto is contained in a housing 30. The metallic stem 10 is fixed to the housing 30 by a mounting block 20, and the diaphragm 11 is distorted by a pressure introduced through a pressure-introducing passage 32.
FIG. 12 schematically shows a vicinity of the diaphragm 11 shown in FIG. 11 in an enlarged scale. A semiconductor sensor chip J40 is mounted on a front surface of the diaphragm 11 via an insulation film 50 made of glass having a low melting point such as silicon oxides. A strain gage J41 formed on the sensor chip J40 converts diaphragm distortion caused by a pressure applied thereto into an electrical signal. The strain gage J41 is made of a P-type region formed on the front surface of an N-type silicon substrate.
As shown in FIG. 11, a circuit substrate 60 is disposed around the sensor chip J40, and the sensor chip J40 is electrically connected to the circuit substrate 60 by bonding wires 64. The circuit substrate 60 is electrically connected to terminals 72 through a pin 66. In this pressure sensor, the diaphragm 11 is distorted by the pressure introduced through the pressure-introducing passage 32. The diaphragm distortion is converted into an electrical signal by the strain gage J41 formed on the sensor chip J40. The electrical signal is fed to an outside circuit such as an electronic control unit of an automobile through the bonding wires 64, the circuit substrate 60, the pin 66 and the terminals 72.
Since the sensor chip J40 is fixed on the front surface of the diaphragm 11 of the metallic stem 10 via the insulation film 50 as shown in FIG. 12, a parasitic capacitance Cp is formed between the metallic stem 10 and the sensor chip J40. In the parasitic capacitance Cp, both the metallic stem 10 and the sensor chip J40 serve as electrodes, and the insulation film 50 serves as a dielectric layer.
The parasitic capacitance Cp causes the following problem when the pressure sensor is used in an atmosphere under high electromagnetic noises. Such electromagnetic noises are generated, for example, by wireless communication devices. An electric current caused by electromagnetic noises (for example, 1 MHz to 1,000 MHz; several volts to 200 volts/m) is transferred from the housing 30 to the strain gage J41 through the metallic stem 10, the parasitic capacitance Cp and the sensor chip J40. This noise current is added to an output signal of the strain gage J41 and is amplified through a signal processor circuit, decreasing a signal-noise ratio (S/N ratio) in the sensor signal or causing malfunction in the system in which the pressure sensor is used.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved pressure sensor in which influence of outside noises on the sensor signal is eliminated or suppressed.
A pressure sensor for detecting a pressure such as a fuel pressure in an automotive injection system includes a metallic stem to which the pressure to be detected is introduced, a semiconductor sensor chip outputting an electrical signal representing the detected pressure, and an electrical circuit for processing the sensor signal. Those components of the pressure sensor are hermetically contained in a housing. The metallic stem has a thin diaphragm that distorts according to the pressure imposed on the rear surface thereof. The sensor chip is mounted on the front surface of the diaphragm via an insulation layer.
The sensor chip is made of a silicon semiconductor chip having a P-type conduction layer and an N-type conduction layer laminated on the P-type conduction layer. The sensor chip is mounted on the front surface of the diaphragm so that the P-type conduction layer contacts the insulation layer. A P-type conduction region is formed in the N-type conduction layer so that the P-type conduction region is electrically separated from the P-type conduction layer. The P-type region formed in the N-type layer constitutes a strain gage, the resistance of which changes according to distortion of the diaphragm. The electrical signal outputted from the strain gage is processed by the electrical circuits connected to the strain gage.
The P-type conduction layer of the sensor chip functions as a shield layer for interrupting outside noises from the strain gage. Preferably, a pad for grounding the shield layer and another pad for grounding the strain gage are provided separately from each other, and both pads are grounded through respective circuits formed separately from each other. By separately grounding the shield layer, the outside noises are effectively interrupted, and the sensor outputs are kept free from the outside noises even the outside noises are in a high frequency region.
The positions of the N-type and the P-type conduction layers in the sensor chip may be reversed so that the N-type layer becomes a base layer and the P-type layer is laminated on the N-type layer. The strain gage may be composed of four strain gage elements forming a bridge circuit.
According to the present invention, outside noises are effectively separated from the sensor outputs by the insulation layer disposed between the strain gage and the metallic stem from which the outside noises are transferred.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.